<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">

<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
  <title>用循环将三个DIV变成红色</title>
  <style>
    #outer {
      width: 330px;
      height: 100px;
      margin: 50px auto;
    }

    #outer div {
      float: left;
      width: 100px;
      height: 100px;
      margin: 0 5px;
      background: dodgerblue;
    }

    button {
      display: block;
      width:200px;
      margin: 10px auto;
      background: #3A64B0;
      color: white;
      border: 0 solid red;
      padding: 10px;
    }
  </style>

  <script>
    window.onload = function () {
      var oDiv = document.getElementById("outer").getElementsByTagName("div");
      var oBtn = document.getElementsByTagName("button")[0];
      oBtn.onclick = function () {
        for (var i = 0; i < oDiv.length; i++) {
          oDiv[i].style.background = "red";
        }
      }

    }
  </script>
</head>

<body>
  <button>点击将DIV变成红色</button>
<div id="outer">
  <div></div>
  <div></div>
  <div></div>
</div>
</body>

</html>
